In certain telephone systems employing so-called floating subscriber lines, a bias network is connected to the tip and ring conductors of each subscriber line keeping the tip and ring at a negative potential as a guard against corrosion. However, in subscriber line testing, which is routinely performed in telephone systems, the bias network tends to interfere with line impedance measurements and to prevent accurate measurement of line leakage. Any attempt to disconnect the bias network from the line during testing, results in the addition of a substantial number of circuit elements for each line, increasing significantly the per-line costs.